It is common in the art of audio signal processing to use adaptive filters to suppress unwanted additive signals, such as noise or echo, from the audio signals that contain desired information such as speech, music and/or other desired information. Such adaptive filters use at least one audio input signal.
Some methods for suppression of unwanted signals may include noise reduction, microphone beam forming and echo cancellation technologies. In some examples, adaptive linear filters can be used to model an unknown linear system that may comprise acoustic or electronic signals. An adaptive filter may be able, in some examples, to estimate the system it models. Adaptive filters are however, in some examples, subject to inaccuracies and performance limitations, due, in part, to a nonlinearity nature of the system.
Practically, most systems are not linear, due to, among other reasons, unwanted and unknown signals. Unwanted signals can include, for example, noise or echo that is added intentionally and/or unintentionally to a desired signal. This addition of uncorrelated noise or echo to the main and/or desired audio signal can be highly nonlinear within the system to which the filter is applied.
In some examples, an Acoustic echo canceller can be used to estimate the acoustic echo leak between, for example, a mechanical speaker and a microphone of a near-end user (e.g., a telephonic device). The echo leak can be described as an unwanted audio signal that was added to a main audio signal. The main audio signal can be, for example, a voice of a human user. The estimated echo leak can be then subtracted from the main audio input signal in order to cancel the real echo leak.
Other causes of nonlinearity within an audio system can include, for example, harmonic distortion. A harmonic distortion can stem from loudspeakers or microphones distortions, or from other sources.
In some examples, a linear system may not be stationary, for example, due to change in the acoustical environment, resulting in a nonlinear behavior of the adaptive filter.
Designers of noise reduction systems and echo cancellers may, in some examples, seek nonlinear solutions that could improve the performance of one or more adaptive linear filters. For example a linear adaptive filter in echo-cancellers can be followed by a nonlinear residual echo canceller. Similarly, microphone beam-forming algorithms can be enhanced by a nonlinear noise gating operation.